Drone-docker plugin

Basic drone-docker plugin work. Feedback appreciated.

Plugins in drone have the ability to modify the buildfile and add commands to be run
after the tests complete. This

This change adds a new Drone plugin for creating a docker image. The plugin
creates a docker image for the test build and pushes the image to images.docker.io.

I've just tested it end-to-end with 'drone build' from the command line because
I'm not sure of another test that would be useful.
the changes in this file seem unrelated to the purpose of the PR, so I'd take them out